Thomas argues in De Ente et Essentia that God is Pure Act, or existence itself subsisting. His metaphysical argument is the ground of the first four of his five ways in proof of God's existence. The argument from motion, the first of these, specifically recalls the transition of a thing's potentiality to actuality. For example, an acorn exists as an acorn in actuality and as an oak tree in potentiality.
Since no potentiality can actualize itself (an acorn needs water, sunlight, etc), it follows that some actuality is needed to actualize a potentiality. Since Pure Act just is existence itself, it's not hard to see why Thomas associates Pure Act with God. All potentialities are possibly actualized, and nothing is actualized apart from Pure Act (existence), so Pure Act ultimately has power over all potentialites and is therefore omnipotent.